Poly(ethylene terephthalate) (PET) films are widely used for a variety of wrapping, packaging, and lamination applications. In other applications such as flexible electronic circuits, touch screen displays, and flexible photovoltaic devices, biaxially-oriented and heat-set PET films having good dimensional stability and shrink resistance at elevated temperatures are used. However, biaxially-oriented PET films are not believed to be useful at temperatures exceeding 200° C. because of their low Tg (˜80° C.) and relatively low inherent melting temperature (Tm) (approximately 250° C.).
It is generally known in the art that biaxially-oriented PET has been used as substrate films for indium tin oxide (ITO) coated transparent conductive substrates. Although adequate for many applications, PET films are believed to lack temperature dimensional stability needed for the high temperature deposition of ITO often necessary to prepare a conductively coated substrate for use in applications where high transparency and high conductivity are desirable, such as in flat panel displays and photovoltaic devices. Higher temperatures are believed to be necessary to reduce the thickness of the ITO coating for a given conductivity. Reduced thickness coatings can resist fracture when bent, and there is a need in many existing and emerging applications for increased durability and flexible form factors while maintaining high transparency and adequate conductivity. These applications include flat panel displays, photovoltaic devices, and flexible displays among others. In addition, it has been reported that for high transparency/high conductivity films requiring a post-deposition annealing process, ITO coated films, where the deposition and annealing were done at greater than 200° C., resulted in conductive substrates with superior conductivity relative to films where the ITO was sputtered and annealed at temperatures less than 200° C.
Biaxially oriented PET has also been used as a substrate for ITO projected capacitive (PCAP) touch sensors for mobile devices such as smartphones and tablets. Personal computers (PC) with touch screens are also gaining popularity. However, ITO transparent conductive film requires heat annealing in order to achieve low enough sheet resistance for acceptable touch sensitivity. The annealing time is normally longer than an hour and the annealing temperature is below 150° C. to prevent deformation of the PET substrate and damage to sputtered ITO conductive layer. Longer and hotter annealing of PET will increase oligomer migration to the film surface and deteriorate the light transmittance of the transparent conductive film. A higher temperature substrate with less oligomer migration than PET is therefore desired. PEN has been suggested as a high temperature substrate. However, oligomer migration to the film surface is also a problem during film heat setting and ITO annealing processes. PCT has higher heat resistance, but crystallizes too fast on the chill drum in extrusion, which results in a brittle film that presents problems for subsequent machine direction and transverse direction stretching, and/or for end use requirements.
Desirable properties of a transparent conductively coated substrate include: transparency, conductivity, flexibility, charge carrier density, charge carrier mobility, tensile and flexural properties, hydrolytic stability, and dimensional stability. The current materials (PET, PEN, polyimide, glass, etc. . . . ) known in the art for use in transparent conductively coated substrates are believed to be deficient in one or more of the aforementioned properties. To achieve superior conductivity while maintaining transparency, flexibility, and substrate durability; a material with greater temperature dimensional stability than PET is needed; a material with improved thickness reduction and flexibility to glass is needed; and a material with improved transparency to polyimide is needed.
Certain applications, such as transparent conductively coated substrates, would benefit from or even require films that are heat stable (i.e., possessing good dimensional stability) at temperatures greater than or equal to 150° C. Specifically, the films should not blister or wrinkle when coated with ITO and/or other inorganic oxide at temperatures greater than 150° C.
Superior hydrolytic stability is another desirable property for films used as conductively coated substrates across a variety of applications. Therefore, base films with superior hydrolytic stability for producing conductively coated substrates, which tend to maintain their structural integrity under high temperature and high humidity conditions, are also needed.